Abstract

We report two novel fabrication techniques, as well as THz spectral transmission and propagation loss measurements of subwavelength plastic wires with highly porous (up to 86%) and non-porous transverse geometries. The two fabrication techniques we describe are based on the microstructured molding approach. In one technique the mold is made completely from silica by stacking and fusing silica capillaries to the bottom of a silica ampoule. The melted material is then poured into the silica mold to cast the microstructured preform. Another approach uses a microstructured mold made of a sacrificial plastic which is co-drawn with a cast preform. Material from the sacrificial mold is then dissolved after fiber drawing. We also describe a novel THz-TDS setup with an easily adjustable optical path length, designed to perform cutback measurements using THz fibers of up to 50 cm in length. We find that while both porous and non-porous subwavelength fibers of the same outside diameter have low propagation losses (α ≤ 0.02 cm−1), the porous fibers exhibit a much wider spectral transmission window and enable transmission at higher frequencies compared to the non-porous fibers.

Figures (6)

Tunable THz-TDS setup for waveguide transmission measurements. a) Schematic of setup. E:Emitter, D:Detector, PM:Parabolic Mirror, BS:Beam Splitter, FM: Flat Mirror, b) Source spectrum (red) and background noise level (blue). There are traces of water vapor (black) despite efforts to purge with a nitrogen atmosphere. c), d) Photographs of a setup for different positions of the mirror assembly that allows to either perform measurements of a point sample c) or to accommodate a waveguide up to 50 cm in length d).

Schematics of various subwavelength fibers and their fabrication techniques. a)-c) Poynting vector distributions across fiber cross-sections for subwavelength fibers featuring 0, 1, and 7 holes, respectively. The outer diameter of all the fibers is 400µm, the diameter of all the holes is 100µm, and the frequency is 0.3 THz (λ = 1000µm). Schematics of the d) sacrificial polymer technique and e) microstructured molding technique for fabricating porous subwavelength fibers.

Transmission and loss measurements of porous and non-porous subwavelength PE fibers. Left column: small diameter fibers, Right column: large diameter fibers. The data for the porous fibers is in red and data for the non-porous fibers is in blue. Fiber diameters and measured segments lengths are indicated in the legends. Photos a) and e) show measured fiber cross-sections; b) and f) Normalized amplitude transmission; c) and g) Power propagation loss calculated from the transmission spectra using cutback technique; d) and h) Upper bound on propagation loss given by normalized (per unit of unit of length) total loss.

Modal losses of the porous and non-porous fibers as a function of the fiber geometry parameters (dF = fiber diameter, P = porosity). First row: non-porous subwavelength fiber; second row: subwavelength fiber with 7 air holes; third row: subwavelength fiber with one air hole. First column: schematics of the fiber geometries; second column: fundamental modal attenuation loss as a function of frequency; third column: power coupling coefficient between a gaussian beam and the fundamental mode.

Theoretical calculations of the dispersion parameter of non-porous (a) and porous (b) fibers. The effective index curves of the non-porous and porous fibers were taken from the simulations presented in Figs. 3.b) and 3.e), respectively. Decrease of the fiber diameter and increase of the porosity result in the reduction of dispersion. Comparison of the time scans of ~450 µm diameter porous (red curve) and non-porous (blue curve) fibers confirms that dispersion is smaller in porous fibers because the length of the dispersed THz pulse is shorter (envelope is decaying faster). The porous fiber time scan is offset vertically and the reference pulse of the source is scaled a factor 1/40 for clarity.

Theoretical fit of transmission spectra through large and small diameter subwavelength fibers. The theoretical fits (solid lines) take into account coupling loss (dotted lines) and absorption loss (dashed lines) contributions but neglect scattering losses. The calculations assumed αmat = 0.2cm−1, a porosity of 35% for the small diameter fiber, and a porosity of 72% for the large diameter fiber. Optimal fits for the non-porous fibers were found for diameters slightly different from the experimentally measured diameters.